The alveoli and distal airways of the lung are lined with an acellular layer of material which is essential for the maintenance of normal pulmonary functions such as gas exchange. The composition and origins of the acellular lining are being investigated. Current attention has been directed towards: (1) the biosynthesis and secretion of pulmonary surfactant and (2) the nature of the cytoplasmic organelles known as lamellar bodies. The objectives of this investigation are as follows: (1) to further develop methodology for the isolation of lamellar bodies from the lungs of rabbits (2) characterize the lamellar bodies according to phospholipid, protein and enzymic components, (3) to elucidate processes involved in the formation of the extracellular lining from secreted lamellar bodies and (4) to determine the distribution of pulmonary surfactant between intra- and extracellular pools.